This visual abstract depics how HIV-1 can escape Cas9/sgRNA-mediated inhibition. The researchers reveal that the NHEJ repair machinery generates mutations in the HIV-1 Cas9 cleavage site that result in two outcomes: viral replication suppression and viral escape.CREDIT Wang et al./Cell Reports 2016

Upon entry into a cell, HIV’s RNA genome is converted into DNA and becomes entwined with the cellular DNA. From here, CRISPR/Cas9 can be programmed to target a DNA sequence and cleave viral DNA. The problem is that HIV is notoriously good at surviving and thriving with new mutations, so while many viruses are killed by the targeted approach, those that escape the CRISPR/Cas9 treatment become more difficult to target.

“When we sequence the viral RNA of escaped HIV, the surprise is that the majority of the mutations that the virus has are nicely aligned at the site where Cas9 cleaves the DNA, which immediately indicates that these mutations, instead of resulting from the errors of viral reverse transcriptase, are rather introduced by the cellular non-homologous end joining machinery when repairing the broken DNA,” says senior study author Chen Liang, Senior Investigator at the Lady Davis Institute at the Jewish General Hospital and the Associate Professor of Medicine at the McGill University AIDS Centre.

“Some mutations are tiny–only a single nucleotide–but the mutation changes the sequence so Cas9 cannot recognize it anymore. Such mutations do no harm to the virus, so these resistant viruses can still replicate, he says”